Our primary research interest is children who are suffering from rare diseases and unexplained diseases of the immune system. The symptoms that the children suffering from rare diseases present vary vastly and can range from being vulnerable to contracting serious and life-threatening infections to organ specific autoimmune diseases or autoinflammatory diseases. A complete phenotyping of the patients is carried out based on individual clinical observations. This type of research based on patient observation is special as every child with a rare disease can be seen as an “experiment of nature”. Due to this analysis based on “systems biology” conclusions about the etiology and pathogenesis of monogenetically caused diseases can be drawn and new, urgently needed treatments can be researched. However, this necessitates good interdisciplinary collaborations. It is of the utmost importance to validate already defined clinical, cellular-biological and genetic phenomenons. It seems to us, as doctors and scientists, that it is very important not to lose sight of the importance of developing new medicines. Cell-based model systems are not only useful for research into pathogenetic principles (such as a system of differentiation for immune cells based on induced pluripotent stem cells), but can also function as surrogate models for the screening of different active substances.